As with most manufacturing businesses, the integrated circuit (IC) industry is driven by customer demands for lower cost solutions without having to sacrifice performance. In the IC industry, one way to reduce cost is to provide attractive solutions that meet existing needs with reduced die area. Of particular interest is the Class-D audio amplification area.
Various approaches exist to implement pulse width modulation (PWM). The pulse width modulation is obtained by taking a stream of pulses and varying their widths as a function of a control input. One form of PWM generation, which is called Natural-PWM (NPWM), involves comparing an analog input waveform with a triangle wave at a carrier frequency of fc. The continuing expansion of digital techniques in the field of audio has led to a different form of PWM generation called Uniform-PWM (UPM), which typically uses a higher order digital modulator to convert a digital Pulse Coded Modulation (PCM) signal to a uniformly sampled PWM signal. In addition to using the sampling method as way to differentiate PWM methods, PWM methods can also be differentiated by the edge modulation and by the class of modulation. For example, Class-AD and Class-BD are standardized abbreviations used to differentiate between two-level and three-level switching. The modulation can also be single-sided or double-sided.
Once the PWM signal is generated, it can be used to drive a high power output FET array, such as a half bridge or an H-bridge. Due to the bi-level nature of the PWM signal for traditional Class-AD, each transistor will be either completely on or completely off. It is the operation of the transistors between on and off conditions that enables the power stage to achieve a much higher efficiency than other types of analog power stages.
Even for an ideal NPWM (and UPWM), a subsequent switching power stage will add noise and distortion due to switching delays that vary non-linearly with load current. The quality of the triangle wave signal can also induce errors. Similarly, noise and nipple on the power supply for the switching power stage will introduce errors in the system output signal resulting in poor Total Harmonic Distortion (THD) and Power Supply Rejection Ratio (PSRR). It is known from delta-sigma converter theory that using an integrator before a noise adding element and then feeding back the inverse of the output pushes the added errors high into the frequency band. If the errors are pushed above the audio band, an external lowpass filter can be used to extract the audio signal from the PWM waveform. The same principle has been used for many existing Class-D architectures. Most of the existing architectures for Class-D amplifiers, however, operate on analog inputs and hence require the presence of a digital-to-analog converter (DAC). Such approaches also require an analog ramp signal at the input of a comparator to generate the PWM output.